1. Technical Field of the Invention
The present invention relates to a method of grinding an axially asymmetric aspherical mirror.
2. Prior Art
A reflecting mirror with an axially asymmetric aspherical surface such as an elliptical surface, parabolic surface or hyperbolic surface (called an axially asymmetric aspherical mirror) is used as an optical element that reflects, focuses or disperses X-rays, laser light, visible light, etc. For instance the mirror with a surface formed by rotating an ellipse shown in FIG. 1A has two focal points F1, F2, and has the intrinsic characteristic that light passing from one focal point F1 is reflected by the elliptical surface of the mirror and travels to the other focal point F2. This elliptical surface mirror also has the characteristic that the mirror converges the light from the focal point F1 into the focal point F2 with high precision. More precisely, as shown in FIG. 1B, a light source with a diameter of 1 mm, for example, located at the focal point F1 is focused by the mirror with a surface formed by rotating an ellipse, into one 200th to 1,000th of the diameter, that is, the light is intensely converged into a spot several microns in diameter. Therefore, these characteristics can be utilized in various applications; for example, the intensity of weak X-rays from an X-ray tube can be increased and used in chemical analysis, soil analysis, etc. using absorption photometry, or a beam of laser light can be converged precisely and used in a laser application such as a laser scalpel.
The necessary conditions for the aforementioned axially asymmetric aspherical surface mirror to achieve the above objectives include the requirements that the shape of the reflecting surface of the axially asymmetric aspherical mirror must be produced with an accuracy of xc2xc or less of the wavelength xcex of the light to be used (for example, 0.3 xcexcm or less), and that the mirror finish must have a roughness of its reflecting surface of 4 xc3x85 (0.4 nm) or less.
However, the conventional means of producing such an ultra-precision mirror surface require a very long time (for instance, several months or more), consequently, this restricts the practical application of axially asymmetric aspherical mirrors, and this is a practical problem.
More explicitly, according to conventional means of processing, the mirror is processed by lapping or by conventional grinding to a surface roughness Rmax of 1xcx9c2 xcexcm (1,000xcx9c2,000 nm), i.e. the practical limit of processing, then the surface of the mirror is finished to the necessary surface roughness (for example, several xc3x85) by polishing. However, the polishing allowance normally required is about 10 times the surface roughness before processing, so, in practice, a depth of 10xcx9c20 xcexcm must be removed by polishing, that is, the processing amount is very large. As a result, for a conventional polishing system in which an elastic deformable tool is lightly pressed onto the surface of an optical element, carefully avoiding damage to the surface, and a slurry containing microscopic grinding grains is used, the polishing time to process a depth of 10xcx9c20 xcexcm can be as long as several months or more.
When an amount of 10xcx9c20 xcexcm is removed by polishing, the residual stress on the surface caused by lapping or grinding is removed, therefore the accuracy of the processed surface with respect to a reference surface becomes worse, and this is another problem. In order to achieve the necessary accuracy in the shape of an ultra-precision mirror surface (xcex/4 or less), the reference surface must be reprocessed after being polished once, and then the polishing and reprocessing should be repeated until the necessary accuracy is obtained. Still another problem is that while repeating these operations, the reference surface of an optical element is often changed.
FIGS. 2A, 2B and 2C shows another example of an axially asymmetrical aspherical mirror, that is a mirror with a rotated elliptical surface in this example. A curved surface with a large radius of curvature is processed on the surface of a rectangular block of raw material (quartz etc.) Therefore if a processing tool, for instance, a pole-nose grindstone is used that rotates around an axis normal to the surface of the raw material (upper surface in FIG. 2C), the processing efficiency at the center of the lower surface is low resulting in an inferior surface roughness. Conversely, if a processing tool, for instance, a cylindrical grindstone is used which rotates about an axis parallel to the surface of the raw material (upper surface in FIG. 2C), the axis of rotation must be long to avoid interference with the raw material, and the accuracy of the process is poor due to the effect of shaft deformation.
The present invention is aimed to solve the above-mentioned problems. In other words, an object of the present invention is to provide a method of grinding an axially asymmetric aspherical mirror with a highly accurate shape, superior surface smoothness and the capability of precisely reflecting or converging light.
According to the present invention, the apparatus is provided with a disk-shaped metal-bonded grindstone (2) with a surface (2a) shaped as circular arc with a radius R on the outer rim thereof, that rotates about an axis Y, an electrode (4) placed opposite the aforementioned grindstone with a space between them, a nozzle (6) that supplies a conducting liquid between the grindstone and the electrode, a device (8) for applying a voltage between the grindstone and the electrode, an electrolytic in-process dressing device (10) that electrolytically dresses the grindstone while a workpiece (1) is being ground, a rotating truing device (12) that rotates around an axis X that is orthogonal to the above-mentioned axis of rotation Y and trues the aforementioned circular arc surface, a shape measuring device (14) for measuring the shape of the circular arc surface of the above-mentioned grindstone and the processed shape of the workpiece (1), and a numerical control device (16) that numerically controls the aforementioned grindstone in three directions along the axes X, Y and Z. The grindstone is moved in the directions of each of the three axes by means of the numerical control device (16), while the operations of truing, grinding and measuring are repeated on the machine.
According to the above-mentioned method of the present invention, the grindstone can be moved in the direction of the three axes by the numerical control device (16), and by means of the rotary truing device (12), the circular arc surface (2a) can be precisely trued on the outer periphery of the grindstone. In addition, by using the electrolytic in-process dressing device (10) that removes metallurgically bonded grinding grains from the surface of the grindstone by electrolytic dressing, as the workpiece is being ground, high-precision processing can be implemented with a high efficiency even with finer grinding grains than are used in conventional grinding methods, without the grindstone becoming clogged. Furthermore, because the shape measuring device (14) measures the shape of the circular arc on the surface of the grindstone after truing and the processed shape of the workpiece (1) after grinding, on the machine, and the data used for processing are compensated according to the measured data and the workpiece can be reprocessed, the preferred shape can be accurately processed while correcting for wear of the grindstone and processing errors.
Another aspect of the method of the present invention is that because the electrolytic in-process dressing device (10), the rotary truing device (12) and the shape measuring device (14) are provided on the same equipment, and the workpiece is mounted on a common installation device, the workpiece can be processed and measured repeatedly without removing it from the installation device, so the reference surface of an optical element need not be reprocessed, and the reference surface is absolutely free from any displacements that might be caused by remounting in a conventional method known in the prior art.
In a preferred embodiment of the present invention, the processing surface of the workpiece (1) is tilted at an angle of between 30xc2x0 and 60xc2x0 relative to the axis of rotation Y of the metal-bonded grindstone (2).
If the diameter of the circular disk-shaped grindstone is made sufficiently smaller than the minimum radius of curvature of the axially asymmetric aspherical surface to be achieved during processing an axially asymmetric aspherical surface according to the method mentioned above, the shaft of the metal-bonded grindstone (2) need not be extended to avoid interference between the workpiece (1) and the axis of rotation of the grindstone, therefore, deflections thereof can be minimized, and a high processing accuracy can be maintained.
Moreover, the surface of the workpiece (1) to be processed is ground by feeding the above-mentioned grindstone in the direction of the axis of rotation Y thereof at a relatively high speed and moving the grindstone in the X direction orthogonal to the axis Y at a relatively low speed.
As a result of the above-mentioned method, it is possible to prevent microscopic elevations and recesses on the surface of the grindstone from being reproduced on the processed surface of the workpiece (1), therefore, the processed surface obtained is excellent in terms of surface roughness.
In addition, a laser-type shape measuring device or a contact-type shape measuring device should preferably be used as the aforementioned shape measuring device.
By using a laser-type shape measuring device, the shape of the circular arc surface of the grindstone and the processed surface of the workpiece can be measured on the machine with a high accuracy from a location some distance away from the machine. On the other hand by using the contact-type shape measuring device, on-machine measurements can be made reliably even under adverse conditions.